Lecture 2 – Abstract Data Type (ADT) CMPT 225 Lecture 2 – Abstract Data Type (ADT)

Learning Outcomes At the end of this lecture, a student will be able to: Define abstraction, information hiding and "abstract data type“ (ADT) Write C++ code Encapsulate methods and variables for an ADT into a C++ class Differentiate between a class that has been designed as an ADT and a class that has not Compare and contrast them

Today’s menu Abstract Data Type (ADT) Example: Temperature class Definition + “Wall” metaphor How to design an ADT How to implement an ADT in C++ How to test an ADT Example: Temperature class Implemented as an ADT Implemented as a non-ADT Compare both implementations

Let’s start with a problem Problem Statement Create a temperature conversion application Step 1 – Understanding the problem What do we do in this step? Step 2 – Design Solution Step 3 – Implement Solution Step 4 – Testing solution

Why is Step 1 is important?

Step 2 – Design solution UML Class Diagram Temperature degrees scale Class Name degrees scale Attributes Operations inFahrenheit( ) inCelsius( ) raise( toWhat ) Exclude constructors, destructors, getters/setters. Why? Because we assume that we will develop these methods for our classes, if they are needed.

Step 3 – Implement solution /* Header Comment Block */ /* Class Definition */ class Temperature { private: double myDegrees; // >= ABSOLUTE_ZERO for myScale char myScale; // 'F' or 'C' bool isValidTemperature( const double degrees, const char scale ); public: }; Header file: Temperature.h Preprocessor and using directives See next slide

Step 3 – Implement solution – cont’d ... public: // Constructors Temperature(); Temperature(double degrees, char scale ); // Getters double getDegrees( ) const; char getScale( ) const; // Setters void setDegrees( const double degrees ); // Application-related methods Temperature inFahrenheit( ) const; Temperature inCelsius( ) const; void raise( const double amount ); }; Constants

Step 3 – Implement solution /* Header Comment Block */ Temperature::Temperature():myDegrees(0.0),myScale('C'){} ... Temperature Temperature::inFahrenheit( ) const { Temperature result; if ( myScale == 'F' ) result = Temperature( myDegrees, 'F' ); else if ( myScale == 'C' ) result = Temperature(myDegrees * 1.8 + 32.0, 'F'); return result; } ... Implementation file: Temperature.cpp Preprocessor and using directives

Step 3 – Implement solution /* Header Comment Block */ int main() { // Display menu // 1. Convert Celsius temperature to Fahrenheit // 2. Convert Fahrenheit temperature to Celsius // Read user choice // Ask user for and read temperature degrees // Create a temperature object of required scale // Convert this temperature object to // temperature object of other scale // Display resulted conversion // Repeat the above until user quits return 0; } TemperatureConverter.cpp Preprocessor and using directives

Step 4 – Testing Temperature class /* Header Comment Block */ int main() { // Create a valid Celsius temperature // Create an invalid Celsius temperature // Create a valid Fahrenheit temperature // Create an invalid Fahrenheit temperature // Converting a valid Celsius temperature // to a Fahrenheit temperature // Raising a valid Celsius temperature to // an invalid amount of degrees ... return 0; } TemperatureTestDriver.cpp Preprocessor and using directives Goals of Test Driver: 1. To test each method of Temperature class by calling it at least once 2. To “break” the code, i.e., calling each method with valid and invalid parameters.

What makes this class an ADT? So, what is an ADT (abstract data type)?

Abstraction – in the real world From the Latin abs, meaning away from  and trahere, meaning to draw “Process of taking away or removing characteristics from something in order to reduce it to a set of essential characteristics” Examples: Car Map Source: https://whatis.techtarget.com/definition/abstraction

Abstraction and information hiding – in the software world We hide the attributes (data members) by listing them as We achieve abstraction by hiding information from “public view” -> from other classes (i.e., client code, test drivers) We separate the purpose (what) of a class from its implementation (how) and hide the latter (i.e., class’ data members and the implementation of its methods) This way, client code can use the class without knowing its implementation (only knowing the class’ interface) This reduces complexity and allows for easy modification And this is how we construct an ADT private within the header: .h file We hide the implementation of its methods by putting them in a separate file: .cpp file public

When Temperature class implemented as an ADT Temperature’s “private” section hidden behind the wall Client code must use Temperature’s public methods (slits in the wall) to access Temperature’s private section Temperature Converter.cpp (Client Code) Temperature class

Why should we hide a class’ implementation? /* Header Comment Block */ /* Class Definition */ class Temperature { public: double myDegrees; // >= ABSOLUTE_ZERO for myScale char myScale; // 'F' or 'C' ... Header file: Temperature.h Preprocessor and using directives Constants

Why should we hide a class’ implementation? cont’d ... // Constructors Temperature(); Temperature(double degrees, char scale ); // Application-related methods Temperature inFahrenheit( ) const; Temperature inCelsius( ) const; void raise( const double amount ); bool isValidTemperature( const double degrees, const char scale ); };

Step 4 – Test solution - Demo Create a valid Fahrenheit temperature -> testing Temperature( 32.0, 'F' ) Actual Result: 'tempFahr' -> 32 degree F Changing the amount of degrees of 'tempFahr' to -976.02F by setting 'myDegrees' to -976.02 directly. Actual Result: 'tempFahr' -> -976.02 degree F

When Temperature class not implemented as an ADT Client code can tamper with the class’ attributes (data members), hence breaking the class invariant In our Temperature class example, client code can break Temperature class’ invariant: Actual Result: 'tempFahr' -> -976.02 degree F Temperature Converter.cpp (Client Code) Temperature class myDegrees myScale

Advantages and disadvantages of ADT Preserve/control the integrity of a class’ data (expressed as invariant of a class) Disadvantages: More code to write: must have getters/setters methods

√ Learning Check We can now … describe what happens in the 4 steps of the software development process: Step 1 - Problem statement Step 2 – Design Step 3 – Implementation Step 4 - Testing construct an ADT class in C++ define abstract data type (ADT), abstraction and information hiding differentiate between a class that has been designed/implemented as an ADT and a class that has not list some of the advantages and disadvantages of ADT

Next Lecture Introduce our first data structure: List Design a List class as an ADT Implement it using an array